The increasing use of electronic media for publication of images and text, for example on the world wide web, has created the risk to the publisher or other copyright owner of loss of control over unauthorized reproductions of the published materials. Web browsers such as the Netscape.RTM. software and the Microsoft.RTM. Internet Explorer software allow the user to instantly copy and store digital images available on the world wide web. Images published on CD-ROM, such as "clip art" and photo image collections, are similarly easily copied. The owners of copyrights in such digital images may in some cases be deterred from making full use of the distribution channels for such images that are provided by electronic distribution on CD-ROM or via the internet and the world wide web. In other cases, the publisher will take that risk, but will charge higher prices to authorized customers to recover some of the losses from unauthorized copying.
It would be desirable therefore to provide a covert digital identifying indicia affixed to digital images, which is minimally detectable to the human eye, so that copies of the digital images could be identified by the copyright owner as reproductions of its images by application of the appropriate detection method. It would be desirable that such identifying indicia be undetectable so that the appearance of the image is not defaced or altered so as to be unusable. Further, it would be desirable for such identifying indicia to be robust, in the sense that copies of the images would retain the identifying indicia despite alteration of the digital image. The identifying indicia should survive lossy compression procedures typical of digital images, for example, such as are associated with the JPEG format for digital images. The identifying indicia should also survive common image modifications possible with graphics software, such as cropping, rotation, resizing, and, in the case of color images, conversion to gray scale images, and still be detectable using an associated detection method. In this way, a publisher of copyrighted images, such as a photo archive or "clip art" publisher, could easily determine if a suspected unauthorized copy in fact was a copy of an image belonging to the publisher, without needing to check a massive collection of images to ascertain if the image belonged to the publisher.
A number of approaches have been proposed to apply identifying information to digital objects, such as graphic images, sound files, or software.
U.S. Pat. No. 5,287,407 to Holmes discloses a method of identifying copies of software by overwriting blocks of data in the software with information about when, and by whom, the copies are made. This method would not be usable in connection with digital images, as it assumes the availability of a blank block of data suitable for encoding copy information. In any event, such a method, if used in a digital image, would require that such information be included at the end of a file, where it could be removed during cropping or other alteration of the image. The data block could not be included in some other portion of the digital file, as it would affect the quality of the image.
U.S. Pat. No. 5,530,759 discloses a method of digital watermarking of graphic images. Digital watermarking involves the affixation of a visible marking to the digital image. The purpose of the digital watermark is to make an image suitable for use in study, but render it unsuitable for copying for republication. The patent describes altering the brightness of select pixels in the digital image to a uniform intensity that defines the watermark, e.g., the brightness of dark pixels are increased, and the brightness of bright pixels are reduced, to provide a clear, visible watermark image. It is to be appreciated that if the watermark is large, it effectively destroys the usefulness of the image by altering it so that details are no longer in their original visible state. But if the watermark is small, then it can be removed by cropping of the electronic image. A further disadvantage of this system is that it requires maintaining separate databases of marked and unmarked images, one for advertising images available for reproduction, or for other study, the other for authorized reproduction.
U.S. Pat. No. 4,776,013 to Kafri discloses a method of encrypting images prior to transmission by encoding the image by a method that selectively substitutes pixel values in the original image grid with pixel values selected from a master grid, a complement of the pixel values of a master grid, or a randomly selected pixel value. This method alters the image so it is not viewable, and would not be usable to encode an invisible identifying indicia, because such indicia would affect the viewability of the original image.
SRA International of Arlington, Va., has developed a method known as ImPrint.TM. that lightens the blue components in an image to affix an identifying watermark. The watermark produced by this technique, and other techniques to alter color components in an image, will not survive conversion to gray scale. Also, it is dependent on the presence of sufficient blue (or other color) components to allow the image to be watermarked.
Digimarc Corporation of Portland, Oreg., has developed a method known as Picture Marc.TM.. This is an adaptive method to modulate variances in an image in proportion to the local variations in an image. This technique does not provide a standard method of affixing an identifying indicia to an image, and detecting the indicia at a later time, as each image must be separately analyzed.
High Water Designs, Inc. of Bedford, N.H., has developed a method known as FBI.TM. that embeds a binary code in image regions with high activity. This method operates in the frequency domain, and the binary code will be lost if the image is rotated by more than about 10 degrees, or if the image is scaled outside a size range of about 80-120 percent, or if more than about 30 percent of the image is cropped.
None of the prior art systems are directed to a minimally detectable identifying indicia that is robust, which survives common data compression methods, as well as surviving cropping, rotation, scaling, and conversion to gray scale, and which still is detectable using an appropriate detection method.